Varactors are commonly used in RF circuits for tuning oscillators, filters and amplifiers.
One problem with varactors is that their capacitance/voltage characteristics are typically very non-linear as shown in FIG. 1a, which illustrates a typical metal oxide semiconductor varactor (MOSvar) capacitance/voltage characteristic. The non-linear feature of the MOSvar is emphasized by FIG. 1b which shows the first derivative dC/dV of the curve of FIG. 1a. 
One device allowing a capacitance/voltage characteristic having an acceptable tuning range and a more linear range to be obtained is a hyper-abrupt varactor. However, the implementation of a hyper-abrupt varactor requires extra processing during manufacture, which is expensive.
An alternative method of overcoming the non-linearity of a varactor is to use digital techniques to switch in capacitors so as to tune over the required range. However, this solution is complex, can be physically large, and may be too slow.
It is an object of the present invention to provide a variable capacitance circuit arrangement having a relatively linear characteristic.